Controlled-grain-precious metal sputter targets

ABSTRACT

A precious metal sputter target has a composition selected from the group consisting of platinum, palladium, rhodium, iridium, ruthenium, osmium and single-phase alloys thereof. The sputter target&#39;s grain structure is at least about 99 percent recrystallized and has a grain size of less than about 200 μm for improving sputter uniformity. The cryogenic method for producing these sputter targets is also effective for improving sputter performance for silver and gold sputter targets.

FIELD OF THE INVENTION

This invention relates to the field of precious metal and precious metalalloy sputter targets and methods for manufacturing the sputter targets.

BACKGROUND OF THE INVENTION

Sputter target manufacturers have relied upon “as cast” as a low-costmethod for producing precious metal sputter targets. In addition tothis, manufacturers have relied upon conventional metalworking andannealing to produce precious metal sputter targets having a moreuniform grain structure. Unfortunately, these techniques provide limitedbenefit for sputter target manufacturers. For example, typical rolledand annealed platinum group sputter targets have a grain size of about300 to 5000 μm.

Target manufacturers have relied upon equal channel angular extrusion(ECAE) to produce fine grain microstructures. Nakashima et al.,“Influence of Channel Angle on the Development of Ultrafine Grains inEqual-Channel Angular Pressing,” Acta. Mater., Vol. 46, (1998), pp.1589–1599 and R. Z. Valiev et al., “Structure and Mechanical Behavior ofUltrafine-Grained Metals and Alloys Subjected to Intense PlasticDeformation,” Phys. Metal. Metallog., Vol. 85, (1998), pp. 367–377provide examples of using ECAE to reduce grain size. ECAE introduces anenormous strain into a metal without imparting significant changes inworkpiece shape. In fact sputter target manufacturers have claimed anability to use ECAE to reduce the grain size of high-purity coppersputter targets to less than 5 μm. Although this process is effectivefor reducing grain size, it does not appear to align grains in a mannerthat facilitates uniform sputtering or provide an acceptable yield—thelow yield originates from the ECAE process operating only withrectangular shaped plate; and thus, requiring an inefficient step ofcutting circular targets from the rectangular plate.

Zhu, et al., in U.S. Pat. No. 6,197,129 B1, entitled “Method forProducing Ultrafine-Grained Materials using Repetitive Corrugation andStraightening” disclose a method for reducing grain size by intenseredundant strains. Unfortunately, the repeated reversing bending strainswould likely produce severe strains at the workpiece surfaces, whileimparting only limited strains to the workpiece's mid-thickness regions,causing a gradient in material microstructure from surface tomid-thickness. These material property gradients are not suitable forsputter targets, as through-thickness property uniformity is a criticalfactor for consistent performance of a sputter target.

Lo, et al., in U.S. Pat. No. 5,766,380, entitled “Method for FabricatingRandomly Oriented Aluminum Alloy Sputtering Targets with Fine Grains andFine Precipitates” disclose a cryogenic method for fabricating aluminumalloy sputter targets. This method uses cryogenic processing with afinal annealing step to recrystallize the grains and control grainstructure. Similarly, Y. Liu, in U.S. Pat. No. 5,993,621, uses cryogenicworking and annealing to manipulate and enhance crystallographic textureof titanium sputter targets.

SUMMARY OF THE INVENTION

The invention provides a precious metal sputter target having acomposition selected from the group consisting of platinum, palladium,rhodium, iridium, ruthenium, osmium and single-phase alloys thereof. Thesputter target has a grain structure that is at least about 99 percentrecrystallized and a grain size of less than about 200 μm for improvingsputter uniformity.

The method of the invention forms sputter targets by first cooling atarget blank to a temperature of less than about −50° C. The targetblank has grains with an initial grain size and a composition selectedfrom the group consisting of platinum, palladium, rhodium, iridium,ruthenium, osmium, silver, gold and single-phase alloys thereof. Thendeforming the cooled target blank introduces strain into the targetblank and forms a deformed grain structure. Recrystallizing the deformedgrain structure forms a target blank having recrystallized grains. Thetarget blank has at least about 99 percent recrystallized grains withthe recrystallized grains having a fine grain size smaller than theinitial grain size. Finally finishing the target blank forms a finishedsputter target at a low temperature sufficient to maintain the finegrain size of the finished sputter target.

BRIEF DESCRIPTION OF THE FIGURE

FIG. 1 is a photomicrograph of fine-grained platinum having a grain sizeof 79 microns.

DETAILED DESCRIPTION

It has been discovered that lowering deformation temperature of preciousmetals and single phase precious metal alloys to at least −50° C. lowersthe temperature of the recrystallization event and results in a finegrain size. For purposes of this specification, precious metals consistof platinum, palladium, rhodium, iridium, ruthenium, osmium, silver andgold. Then heating the target blank to a recrystallization temperatureless than that normally used for ambient-temperature worked metalstabilizes the fine microstructure with a minimum amount of graingrowth. This process produces a fine-grained-recrystallized structurehaving excellent stability at temperatures encountered duringsputtering.

In particular, the process for manufacturing the precious metal targetsfirst introduces severe plastic straining at cryogenic temperatures withthe intent of increasing the number of viable new grain nucleation sitesfor subsequent activation during a low-temperature recrystallizationannealing treatment. This increases the number of nuclei (N) fromintense plastic deformation, reduces the subsequent growth rate (G) ofthe new grains and results in a reduced recrystallized grain size.

The cryogenic process exploits reduced grain boundary mobility byforcing the recrystallization event to occur at low temperatures. Hence,cryogenic working maximizes the ratio of N to G by both the intenseplastic straining and retarded dynamic recovery associated withdeformation at cryogenic temperatures (increasing N), and the reducedgrowth rate of newly formed grains by allowing recrystallization tooccur at lower temperatures (reducing G). Maximizing the ratio of N to Gallows minimization of the recrystallized grain size. Then controllinggrain growth during subsequent processing of the target blank into afinished sputter target maintains the resulting minimum grain size.

The broad application of lower-than-normal deformation temperatures byimmersing target blanks into cooling baths immediately prior to formingoperations achieves a highly worked deformed state. Then annealing atlow temperatures, produces new fully recrystallized grains of relativelysmall size that replace the deformed grains.

Most advantageously, the method includes the optional steps of firstdeforming the target blank to store the requisite energy required torecrystallize the precious metal. And second, recrystallizing the targetblank improves uniformity of the grain structure before initiating thecooling, cryogenic working and low-temperature annealing sequence.

This process produces both precious metal and single-phase preciousmetal alloy sputter targets having at least about 99 percent of thesputter target recrystallized. For purposes of this specification, highpurity refers to precious metal having a purity of at least 99.9 percentby weight; and single phase alloys refer to precious metal alloyscontaining predominantly a precious metal. This specification refers toall compositions by weight percent, unless specifically expressedotherwise. This process is effective for precious metal targets having apurity of at least 99.9 weight percent. In addition, this process isuseful for targets having a purity of at least 99.99 weight percent andmost advantageously as high as 99.9999 weight percent. For single phaseprecious metal alloys, the alloy most advantageously contains less thanten weight percent non-precious metal constituent by weight percent.

The finished grains for the platinum group metals (platinum, palladium,rhodium, iridium, ruthenium and osmium) have a grain size of less thanabout 200 μm. This represents a significant improvement in grain sizeover conventional platinum group metal targets. Furthermore, thisprocess can advantageously maintain grain size to levels less than about100 μm for the platinum group metals. Most advantageously, this processmaintains grain size at levels of about 0.3 to 90 μm for platinum groupmetals. Cryogenic processing generally produces finer grains for silverand gold sputter targets than that achieved for the platinum groupmetals.

For purposes of this specification, orientation ratio defines therelative proportion of a particular grain orientation in relation tototal grains, expressed in percent as measured perpendicular a sputtertarget's face. For example, measuring the intensity of an x-ray peak anddividing it by the relative intensity of that peak measured in a randomorientation powder standard calculates grain orientation ratio. Thisratio is then multiplied by 100 percent and normalized, i.e. divided bythe sum of all grain orientation ratios between the intensities andtheir corresponding relative intensities.

For face centered cubic precious metals, such as platinum, palladium,rhodium, iridium, silver and gold, the finished sputter target faceadvantageously has a grain orientation ratio of at least about tenpercent of each of the (111), (200), (220) and (311). Mostadvantageously, the finished sputter target face has a grain orientationratio of at least about fifteen percent of each of the (111), (200),(220) and (311) for the face centered cubic precious metals. Thisbalanced combination of (200), (111), (220) and (311) orientation ratiosprovides the most uniform sputter properties. In addition to this, theface centered cubic target advantageously has at least thirty percent(200) for improved sputter performance. And most advantageously, it hasat least forty percent (200) or close packed direction.

First cooling a high-purity target blank to a temperature of less thanabout −50° C. prepares the blank for deformation. The cooling medium maybe any combination of solid or liquid CO₂, liquid nitrogen, liquidargon, helium, or other supercooled liquid. Advantageously, the processlowers the blank to about −80° C. Most advantageously, the process coolsthe blank to at least about −196° C. or 77 K. The most practicaltemperature for most applications is 77 K (liquid nitrogen atatmospheric pressure).

After cooling, deforming the cooled high-purity target blank introducesintense strain into the high-purity target blank. The deforming processmay include processes such as, pressing, rolling, forging to form adeformed grain structure and to subsequently achieve fine grain sizesupon low-temperature annealing. During deformation, it is important tolimit heating of the target blank. Furthermore, it is advantageous toenter an engineering strain of at least about 50 percent into the targetblank. This strain ensures uniform microstructure through the target'sthickness.

Rolling has proven to be the most advantageous method for reducing grainsize and achieving the desired texture. In particular, multiple passrolling, with re-cooling to cryogenic temperatures at least once betweenpasses provides the most advantageous results. Most advantageously, there-cooling occurs between each pass. But for some applications,re-cooling after every second pass is sufficient.

The grains in the target blank recrystallize at a temperature that islower than that exhibited by grains that have been worked at ambienttemperatures. At these lower recrystallization temperatures, still atleast about 99 percent of the grains recrystallize. As discussed above,minimizing the recrystallization temperature reduces the target's grainsize. Advantageously, the recrystallizing occurs at a temperaturebetween about 150 and 500° C. For platinum sputter target blanks, therecrystallizing advantageously occurs at a temperature between about 400and 550° C. For palladium sputter target blanks, the recrystallizingadvantageously occurs at a temperature between about 500 and 650° C.

The finishing of the high-purity target blank into a finished sputtertarget occurs at a temperature sufficient to maintain the fine grainsize. If the sputter target is finished at too high of a temperature,then the beneficial grain size reduction is lost. Advantageously, thefinishing occurs at a temperature less than about 200° C. to limit graingrowth. Reducing finishing temperature to less than about 100° C.further decreases grain growth during finishing. Most advantageously,the finishing occurs at ambient temperature.

EXAMPLE 1

A billet of 99.99% pure platinum was cast into a 89 mm (3.5 in)diameter×152 mm (6.0 in) long cylindrical graphite mold. The billet washot upset pressed 67% reduction in height after being reheated to 800°C. The pressed billet was then reheated to 800° C. and hot cross-rolledto a thickness of 38 mm (1.5 in) and allowed to cool to roomtemperature. The slab of platinum was then further cross rolled atambient temperature to a final thickness of 17 mm (0.650 in). Theambient-temperature-rolled slab was annealed at 600° C. for two hours tofacilitate recrystallization. The recrystallized blank was thensubjected to cryogenic cross rolling. In the cryogenic cross rollingstep, an operator immersed the workpiece in liquid nitrogen untilvisible boiling of the liquid nitrogen was no longer observed; theworkpiece was then at a temperature of approximately 77 K or −196° C.Re-cooling the cryogenically rolled slab between each rolling passensured that the imposed deformation took place at a temperature asclose to −196° C. or 77 K as reasonably possible.

After pre-cooling was complete, rolling the platinum workpiece at 1 mm(0.04 in) per pass was conducted until reaching a final thickness of 51mm (0.20 in). In between rolling passes, immediately transferring theworkpiece into the liquid nitrogen bath prevented the temperature of theworkpiece from exceeding approximately −80° C. This facilitatedretaining the maximum stored strain energy imparted by the rollingpasses. In addition, rotating the target blank ninety degrees with eachpass or “cross rolling” facilitated the formation of a fullyrecrystallized and balanced grain structure after annealing.

A recrystallization heat treatment at 475° C. for 2 hours causes thedeformed structure of the intensely strained workpiece to be replacedwith new, relatively fine grains depicted in FIG. 1 (grain size of 79microns). The orientation ratios measured from the fine-grain platinumsample of the present example are listed in Table 1 below.

TABLE 1 Orientation Ratios of Fine Grain Platinum <111> <200> <220><311> 15.2 47.6 15.6 21.6

EXAMPLE 2

Using a process similar to that described in Example 1, a billet of99.99% pure palladium was subjected to thermomechanical processing thatincluded cryogenic deformation and low temperature annealing. Thewrought palladium sample was cryogenically cross rolled as described inthe previous example, using a bath of liquid nitrogen as the coolingmedium. After 66% cryogenic rolling strain, the sample was annealed forone hour at 600° C. The finished material grain size was fullyrecrystallized and measured to be 33 microns, using ASTM E-112-96 grainsize determination procedures.

The process can fabricate targets of any shape including circular-shapedtargets and sheet-like-rectangular-shaped targets. With the cryogenicprocess, it's possible to achieve minimum grain sizes as fine as 10 to100 μm in pure precious metal targets, such as platinum and palladiumtargets having a purity of at least 99.99 weight percent. Furthermore,reducing grain size improves sputter uniformity in comparison toconventional high-purity sputter targets that are most-often annealed athigher temperatures. In addition, the process provides a more consistentproduct than conventional wrought methods. Finally, the target containsa fully recrystallized-textured grain with a balanced orientation ratiothat further facilitates uniform sputtering.

Although the invention has been described in detail with reference tocertain preferred embodiments, those skilled in the art will recognizethat there are other embodiments of the invention within the spirit andthe scope of the claims.

1. A precious metal sputter target, the sputter target having a platinum alloy composition and a grain structure, the grain structure being at least about 99 percent recrystallized and having a grain size of less than about 200 μm and a face centered grain orientation in the (111), (200), (220) and (311) directions for improving sputter uniformity, the sputter target has a grain orientation ratio of at least about 10 percent each of the (111), (200), (220) and (311).
 2. The sputter target of claim 1 wherein the sputter target has a purity of at least 99.9 weight percent.
 3. A precious metal sputter target, the sputter target having a platinum alloy composition and a grain structure, the grain structure being at least about 99 percent recrystallized and having a grain size of less than about 100 μm for improving sputter uniformity the sputter target has a sputter target face and a grain orientation ratio of at least about 10 percent each of the (111), (200), (220) and (311).
 4. The sputter target of claim 3 wherein the sputter target has a purity of at least 99.9 weight percent. 